Chemical composition of organic polymers make them easily combustible. A wide variety of flame retardant additives is therefore usually blend into polymers, to achieve stringent flame retardant standards, demanded by processors and even by national/international legislations.
Recently, despite the fact that halogen free flame retardant agents/additives (FR) represent, generally speaking, solutions that are more expensive compared to traditional brominated flame retardant agents, some preference has been given to halogen free flame retardant additives because of environmental as well as safety reasons. Therefore, halogen free flame retardant agents (FR) are of increasing interest in the thermoplastic polymers market. Basic requirements for these products are good processing performances in compounding and moulding conditions, good mechanical and electrical properties in the solid state, no blooming or discoloration during moulding or later on, good flame retardant properties in both reinforced and unreinforced polymers.
Pure polystyrene is fairly hard, but brittle. Higher impact performances products may be obtained by modifying them with rubbers, such as, for example, polybutadiene rubber. Accordingly, impact modified styrenic polymers are obtained.
Impact modified styrenic polymers can be described as a continuous, rigid styrenic thermoplastic continuous phase containing dispersed rubber particles as additional phase. The rubber phase may be difficult to disperse finely into the thermoplastic continuous phase by melt mixing into extruders, because of different interfacial surface tension of the two polymers. Impact properties of products containing well-dispersed rubber are generally much better than impact properties of products containing rubber not well-dispersed.
Adding rubber during styrene phase polymerization may form some graft copolymers, which being chemically bond to polystyrene phase, may act as homogenizing for the rubber itself. Polymerization may be done in the monomer mass, emulsion or suspension.
Impact modified styrenic polymers are also known as rubber modified styrenic polymers and some well known examples include high impact polystyrene (HIPS) and acrylonitryle butadiene styrene copolymer (ABS).
Impact modified styrenic polymers are widely used in electronics and consumer goods such as computer consoles, televisions, cellular phone, computers, stereos, toys and many other.
HIPS and ABS are commercially avaliable and are produced by well known methods.
Flame retardant HIPS and ABS are achieved using brominated compounds as flame retardant agents, because of their ability to keep good mechanical properties of the polymers, such as impact resistance. Among the most commonly used flame-retardant agents for ABS and HIPS, are etylene bis(tetrabromophtalimide), decabromo di phenylethane, brominated epoxy oligomers and tetrabromo bis phenol A. Antimony oxide may be used as synergist. Brominated compounds however need to be replaced, owing to progressive restrictions in the use of halogenated flame retardants.
Some commercial non-halogen flame retardant alternatives used in HIPS are, for example, organic aryl phosphorous compounds such as:    Resorcinol bis (biphenyl phosphate)    Bis phenol A bis (biphenyl phosphate)    Polymeric biphenyl phosphate    Diphenyl cresyl phosphate    Triphenyl phosphate.
These organic aryl phosphorus flame retardant agents, however, are not able to meet the most stringent criteria in term of flame retardant performances, i.e. UL-94 V0, indeed resulting successful only as UL-94 V2.
Polymer producers have been, till now, unsuccessful in trying to obtain halogen-free HIPS and ABS flame retardant agents which would meet the highest standards of flame retardancy, i.e. UL-94 V0.
HIPS flame retardant halogen free agents, meeting the highest flame retardancy standards, are made by blends of high impact styrene polymers with Phenyl ether polymers, and are commercially available. Phenyl ether polymers are characterized by high level of inherent flame retardancy. These polymer compositions are simply called “HIPS” or more correctly “HIPS/PPO”. Commercial phenyl ether polymers belong to two different chemical classes: polyphenyl ethers (PPE) and polyphenylene oxides (PPO). PPE and PPO are similar in chemical composition, they are generally treated as equivalent materials, and generally, both referred to as PPO. They are polymers difficult to process, and despite blending with Polystirene or HIPS make the polymer composition easier to process, many difficulties are still faced in molding operations, compared to pure HIPS resins. Moreover, PPO is generally more expensive than HIPS resins themselves. HIPS/PPO blends are not objective of the present invention.
ABS flame retardant halogen free products meeting the highest flame retardancy standards are made by blending ABS polymers with Polycarbonate, and are available from major producers. Polycarbonate is characterized by high level of inherent flame retardancy. These compositions are called “PC/ABS”. Polycarbonate is a polymer difficult to process, and despite the fact that its blending with ABS makes the composition easier to process, some difficulties are still faced during molding operations, in comparison to pure ABS resins. Moreover, polycarbonate is generally more expensive than ABS resins themselves. PC/ABS blends are not according to the present invention.
CN 102746608A discloses an environment-friendly flame retardant ABS resin composed by an aluminum hypophosphite and an at least an auxiliary flame retardant chosen from polyehtylene wax, calcium stearate, pentaerythritol, melamine cyanurate and ammonium polyphosphate. However, these compositions lack of sufficient flame retardant properties, particularly they do not reach V0 classification according to UL-94 standard on 1.6 mm thickness.
CN 103113708A describes an ABS flame retardant resin where the flame retardant agent is an organic phosphinate salt or hypophosphite salt and a synergic ammonium salt. However, also in this case, these compositions lack of sufficient processability, giving rise to flames and smoke during the compounding step.
There is indeed a need for highly flame retardant compositions based on polystyrene polymers, characterized by high flame retardant properties and satisfactory process stability.